With a tank dkh at 11 and your IO water at 16, doing a 20% water change will bump your dkh up to 12, only a single degree. I regularly raised my dkh by a degree in a single dose in a heavily loaded sps/lps tank without concern and have had my dkh up at 15 without observing negative impact on any critters. I would not be concerned with doing a full 20% water change. As for cutting your dkh down, liquid phosphate destroyers such as Brad's (Tailored Aquatics) primarily react with phosphate and once depleted, start to pull your dkh down. If I recall correctly, with no phosphate in my tank, one dose of Brad's stuff pulled my dkh down about 1 degree, but again, I don't see the need to do this in your situation.
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